Celestia cast teary purple eyes down over her little sisters. "But, mother! Father! None of us can leave! It would just be..."
"...Awful." Luna finished softly from between her big sister's hooves, where she was nuzzling the downy white coat. The little filly shook her cobalt fur out as if it made her seem bigger in the face of their choice.
Andromeda sat silently next to her sisters, running her hooves through her star-flecked mane. Her nearly kohl eyes watched the little specks of light twinkle away in her long tresses, understanding shining back from the depths of her pupils. Her shadow shimmered beside her, pinpricks of light sparkling on the throne room floor where the shadow of her mane stretched in the evening Sol, like pieces of her simply were not there.
King Cosmos sat upon his dark throne, addressing not his daughters, but his students. "Your mother and I understand," Queen Galaxy nodded solemnly where she was perched on her pearly throne, "that this will be, perhaps, the hardest decision of your lives. But it must be made."
"Verily." Galaxy cast a shimmering gold hoof upwards, towards the glass ceiling through which the sky shone, divided evenly by the day and the night. "For it is time to move the days again, and for Sol and Moon to dance across the sky once more. We must pass the torch on to you, my dearest darlings. But it is not only these that need guiding."
"The stars." Andromeda's small, bell-like voice spoke when the rest fell silent. "I... Can be the stars."
The crushing vacuum of the echoing nothing that replied weighed heavily on them all.
"I love the stars." She stopped playing with her mane, letting the locks cascade back over her shoulders. "Not as much as I love my sisters, but I love the stars. I can be with them."
"Andromeda..." Celestia let out a shuddering breath, holding back her tears still. "You don't have to go."
"I know." She whispered. "But I can."
"Come, little ones, and watch your sister become one with the stars." Galaxy led her remaining daughters up the tower stairs to the observatory. She adjusted the golden telescope, pointing it to follow Cosmos and Andromeda into the furthest reaches of the sky.
Each filly took turns watching as their sister's dark pelt became harder to focus on, giving way to the starlight, until she faded from view entirely, only the two glistening orbs of her eyes remaining against the stars.
Andromeda was beside herself with giddiness, galloping through the asteroid fields on the outside of the Celest system. Today was the day she gets to see her sisters for the first time in a century, finally making her rounds again towards the pony planet.
Through her millennia of living in the dead quiet vacuum of space, her magic had adopted new properties, changing the makeup of her body until she was no more than an illusion from a fourth plane, carrying twinkling stars from unrecognized universes in her mane and fur. She could interact with and change the course of the stars she tended to without harming herself, becoming immersed in degrees of Kelvin off the scales she'd known in her schooling on Equestria, from fifty thousand to absolute zero. She moved across space on silent hooves, propelling herself through the lower dimensions at will, unbowed to the laws of physics and the speed of light.
But she'd slow down today. Just to pass by her sisters and show them that she was still out there, still holding her part of their triad.
She basked in the warmth of the yellow Sun as she approached, allowing more of herself to materialize on the third plane until she was solidly visible. The edges of her mane trailed light, like a comet, leaving icy stardust in her wake. I will shine brightly for my sisters today.
Slowing beyond the speed of light, she came upon the green and blue planet and allowed herself to be pulled into its relatively weak gravity. The pull of the mass center brought out more stardust from her mane, until she glowed as bright as a star would in the Equestrian sky.
Celestia and Luna met at the old telescope that night, looking through the pristine kept glass to see their sister, galloping across their sky with a smile on her starry muzzle. Even though they knew that she could not see them, they waved at her and shed tears for their beloved sister.
"She visited me on the moon." Luna reminisced quietly. "When nobody else could, she came and talked to me. She would bring heat from the faraway stars to warm me beneath the surface. She never forgot about us, sister."
Celestia nodded. "I saw. I would watch her from here, sitting on the surface of the moon and talking all alone. She would stop and listen for you to say something, but just keep going. For hours." Her smile was sad.
"I heard everything. She would talk about all the stars she tended to, about the ones that were dying or ready to go nova. How she would trigger novas in dead systems to restart the circle of life." Luna shook her head. "She was so busy, and yet she still came to talk to me every hundred years."
"She is a beautiful soul."
Passing by the kingdom again, her third crossing of the planet, Andromeda started to build up speed for escape velocity. Goodbye, my sisters. I shall see you again. Her hooves galloped over the tiny satellites until her speed built up to the point that her physical form had to melt away, leaving only the starlight of her mane and tail in her wake, a long stream of twinkling light fading out behind her.
